There has recently been increased concern regarding asbestos-containing products. These are considered to be potentially carcinogenic. This has led increasingly to product substitution and a marked increase in safety measures where people may be exposed to asbestos. Asbestos in fine particulate form is thought to be especially dangerous as it may become airborne and be inhaled. Concern about the potential carcinogenicity of asbestos has resulted in decreased popularity of asbestos-containing products in refractory and thermal insulation materials. Feed tips of the present invention are refractory materials without asbestos.
Reasonably satisfactory molten metal feed tips have been made from an insulating composition that is a composite of asbestos fibers and clay particles. Such a material is available from the Johns Manville Company under their trademark Marinite. This material is obtained in flat slabs or planks which are cut and machined to the desired configuration for molten metal feed tips, as well as various launders and other parts of the molten metal handling system.
Although such material is readily formed and resists molten aluminum and the like it has significant shortcomings. One of the more significant problems with such material is the presence of asbestos.
Other insulating materials have been substituted for asbestos in refractory materials. One such effort is represented by U.S. Pat. Nos. 4,232,804 and 4,303,181.
In U.S. Pat. No. 4,232,804 a molten metal feed tip for a continuous caster comprises a pair of generally rectangular refractory members spaced apart for forming a metal feeding gap between the members. Each of these members has a downstream edge and an upstream portion having a greater thickness than the downstream edge and each is formed of a layer of refractory fibers such as an alumina-silica composition, rigidly bonded together by a refractory binder containing colloidal silica. The members can be laminated to provide non-homogeneous properties between the inside and outside faces. A glass fabric may be bonded to the inside faces of the members forming the feed tip.
A tip as described in U.S. Pat. No. 4,232,804 has appreciable advantages over that previously formed of an asbestos and clay composition. Prominent of course is the elimination of a health hazard. In addition, however, the feed tip is roughly twice as strong as that previously used and is essentially unchanged by contact with moisture. Tips made of refractory fiber felt bonded with colloidal silica can be reused in the continuous caster whereas the prior tips containing asbestos could rarely, if ever, be reused.
In U.S. Pat. No. 4,303,181, a feed tip as described in U.S. Pat. No. 4,232,804 is formed of a felt of refractory fibers rigidly bonded together by a refractory binder. The downstream edges of the members providing the feed tip are compressed to a higher density and strength than the upstream portion, and have a higher thermal conductivity than the upstream portion.
Similarly, the feed tip described in U.S. Pat. No. 4,303,181 is roughly twice as strong as that previously used and is substantially unchanged by contact with moisture.
A new refractory material which we have found suitable for forming feed tips for the casting of molten metal has recently been marketed.